Warburton stood down as Wales captain before the Six Nations Championship, but stepped back up again for the Lions, despite thinking the call from head coach Warren Gatland was a prank.
"I was in a supermarket car park back home," he said, at a lavish squad announcement in Brentford. "My wife was just in the shop, getting some bread and milk.
"I didn't want to go in, because there is a cashier there who always wants to talk about rugby. I actually hung up on 'Gats' first of all when I got the call."
Later, he elaborated: "I was in the gym and my phone was unlocked, so when I was driving home, I thought, 'Surely the boys have been in my contacts, which they always do, and changed their names to Gats'.
"It was Tom James to be exact, the Welsh winger. I thought, 'I know TJ has changed his name to Gats', so when it rang and I didn't hear anything, I hung up, because I thought, 'It's him, messing around up in Merthyr', where he's from.
"Then it rang again, and I heard Gat's Kiwi accent and said, 'Sorry mate, I thought it was TJ messing around'."
Only the second two-time Lions captain, Warburton thinks his best chance is to be picked at No 7 (openside flanker). He sustained a medial knee ligament strain, when Cardiff Blues played Ulster earlier this month, but is expected to return within six weeks.
He was picked as Lions captain again for his experience, gift for communicating with referees and good Six Nations form. Howley says: "He's certainly a leader by actions. I was speaking to Paul O'Connell last night. He [Warburton] doesn't say too much. Johnno [Martin Johnson] was the same.
" 'Just go and do it' - those were the famous words with Johnno. Sam is like that and he's a world class seven.
"Warren Gatland was conscious of competition for places. When you're looking at the captaincy, nothing is given, but if Sam plays as well as has done in the championship and continues that form into the Lions series, then it's going to be an interesting final discussion in terms of selection.
"Having worked alongside Martin Johnson as a player, I'd say the qualities are quite comparable. There's a presence about Sam.
"He certainly leads by example. His way of dealing with referees is quite important."
Warburton was asked what this meant.
"Not overdoing it", was his reply.
He said: "I was having a conversation with Nigel Owens [the Welsh referee] and saying, 'Surely if a player comes up to you 10, 12 times a game, it's going to bug you', not just in refereeing, but in any walk of life?
"It has more substance, if you can go up perhaps five or six times a game. They're more likely to listen to you then ... it's picking your moments."
In conversation, he manages to radiate both warmth and authority, two qualities that render his appointment logical.
"I'm a lot more relaxed this time round than I was four years ago, because I half know what to expect," he said. "I'm just thrilled with the honour of doing it twice.
"I genuinely didn't think I would be a front-runner for it. I thought it would be one of the captains from the home nations.
"I talked myself out of it all the time. Surely, it wouldn't be me.
"Which is why the phone call was so, so surprising, but it was a no-brainer. Because I'm playing well, I feel a lot more comfortable to take on the captaincy, while I didn't quite feel in that place back in January, prior to the Six Nations."
Bonding, he says, will be the easy part.
