When Mad Men's fourth season came to an end all the way back in 2010, loyal fans were left reeling by Don Draper's surprise proposal to his young secretary Megan Calvet, a decision which to many viewers came entirely out of left-field.

In truth, as Jessica Paré, the likeable 29-year-old Quebecoise actress who plays Calvet, explains ahead of the show's long-awaited return this month, Draper's decision was not as out-of-the blue as it seemed.

"I think the thing about Megan is that she says to Don, 'I don't care, I love you'," says Paré. "I can certainly see why Don asks her to marry him. She's good with his children. She's also young enough that she is willing to accept him, both the good points and the bad. He might not be perfect, but he's perfect to her. I think that level of unconditional acceptance is a wonderful feeling for anyone."

Put like that, it also makes sense that ladies man Draper would prefer to marry a much younger, possibly naive, girl, instead of committing to Faye Miller, the doctor who knows the truth about his hidden life. As the ever-wonderful Joan Holloway icily remarked on hearing the news: "He's smiling like a fool, like he's the first man who ever married his secretary." Show creator Matthew Weiner put it yet more succinctly at a recent press conference: "She's 25-years-old.

He gets to go back to being married to a person who sees him as he wants to be."

It's a decision that feels true to Draper's character. Anxious viewers may wish Draper would make mature relationship choices, but the reality is that asking his much younger secretary to marry him after a surprise holiday in California is exactly what the twice-married ad exec would do. It's a decision that feels both rooted in its time and in the character, and yet another reason why Mad Men remains so compelling to watch.

Where lesser shows would probably have moved to solidify Draper and Miller's relationship, Weiner instead chose to illustrate why the Don Drapers of this world prefer to plump for the easier option.

"I think a lot of people were definitely disappointed that he picked the easier woman," admits Paré with a laugh. "Faye was challenging and demanded the best version of him. She wanted him to grow up and evolve but then Faye was also the person who foreshadowed the whole proposal to Megan when she said, 'you'll be married within the year'. She might hope that Don would become his better self, but she knows him."

There are those who felt Calvet waged a stealth campaign to win Draper's hand, arguing that she's more like Roger Sterling's manipulative young wife Jane than might first appear. "I don't think that's true," says Paré. "She doesn't seem to me to be conniving or manipulative or have an agenda... of course that could all change."

Certainly Draper's decision to marry her looks set to drive the new season's plot in multiple ways. For, as the final episode of the last season made clear, while Draper sees the chance to start afresh with an uncomplaining young wife, Sterling sees Draper becoming more like him, and Peggy Olson and Holloway see a declaration of war. The last season ended with an increasingly disillusioned Olson wondering what was wrong with a world where Draper's proposal was celebrated more than her saving the business. Weiner has stated that the new season's theme is "every man for himself".

As to whether Calvet actually becomes the second Mrs Donald Draper (and the third Mrs Dick Whitman), Weiner was at pains to stress that the couple have not yet married, and Paré remains tight-lipped despite leaked paparazzi shots of the two together. "I can't confirm or deny anything about the new season," she says. "They're engaged but I can't tell you if the wedding has [happened] or will actually happen."

Such on-set secrecy is nothing new for Paré, who admits that one of the bigger challenges of working on Mad Men was being given no back-story for her character ("It was interesting, I didn't really know where she was going and I didn't want to act my way out of a job and do more than was on the page") and says that she was as surprised as anyone to see Calvet elevated from front-desk receptionist to secretary to love interest.

"Surprised is actually putting it mildly. There was a joke on set because someone from costume had read the next script, and it said, 'Megan kisses Don', so everyone was saying, 'you're on the way out then', because the women Don kisses don't usually have a great shelf life," she says. "Everyone kept saying, 'well it was nice working with you'. To be honest I was just pretty excited to have any kind of storyline at all... when I first auditioned all that they really knew about Megan was that she was this sort of vague dark-haired character."

The dark-haired reference is interesting because thus far Calvet appears to stand in relief to Draper's icy blonde ex-wife Betty, a woman for whom the word "froideur" is almost too friendly. Where Betty is openly contemptuous of her children, Calvet is easy-going around them. Where Betty craves Draper's approval, Calvet sunnily expects it. Where Betty is desperately grasping for her position in the world, Calvet seems assured that she has one. The age gap between the two women is both a few years and a generation.

"It's telling that when Don and Megan kiss, she [Calvet] says, 'I'm not going to run out of here crying tomorrow', and she means it," says Paré. "There's no agenda, she's more a modern girl in many ways - I think that she fully expects to have a career and that the secretarial job is a step on the way to that. She shows an interest in advertising, she's not planning to be a career secretary. I do think there's a suggestion that, because she's that little bit younger, she's that little bit more free than Peggy, Joan and Betty. She's wide-eyed and gung-ho but there's a confidence there."

Paré herself is refreshingly realistic about her role on the show, admitting, "I could be gone by the next script". She knows both what it is like to be hailed as the next best thing - as a wide-eyed 18-year-old she had the lead in respected French-Canadian director Denys Arcand's film Stardom - and to spend long periods of time supporting yourself with television guest appearances (after Stardom she had an eye-catching turn in 2004's Wicker Park followed by bit parts until landing 2010 comedy Hot Tub Time Machine and Mad Men).

"Just having this job is more than I've been able to say for a long time," she says. "What I learned after Stardom was that making it in acting doesn't take one big break, it takes a series of them. For people who don't have to audition and can finance their own projects it must be a lovely place to be but that's only a very small percentage of acting people.

"When I wasn't working I knew I had to keep doing things, so I spent a lot of time trying to write, although I'm not very good at it, then I took up knitting, I painted, I taught myself to crochet and tried gardening, took up cooking... it was good but obviously I don't like being unemployed." And if Calvet's future does turns out to be short-lived, Paré remains grateful for her time on set.

"I'm actually just thrilled to be a part of this show, working with an actor who's as generous and conscientious as Jon Hamm. There's no real downside for me. It goes without saying that I am incredibly lucky not just to be working but to be working on Mad Men. It's one of those things that will always be a feather in your cap."

Who: Jessica Pare as Megan Draper
What: Mad Men fifth season
Where: SoHo
When: Double episode premiere Saturday April 14, 8.30pm. Encores Wednesdays 7.30pm from April 18

- Independent