Their comedic chemistry, honed together since their late teens, is just plain fun to watch. They play off each other so naturally and effortlessly that their always-increasing awfulness to each other just feels completely real. No matter how ridiculous things got.
And things often got very ridiculous. Nevertheless, it felt entirely plausible when, say, in a late season fight over a shared romantic interest they both kept grabbing hold of an electric fence in order to pass the painful shock onto the other in a futile and agonising attempt to gain the upper hand.
If Peep Show was prepared to bet that people would be willing to watch these two squabble and engage in juvenile battles of one-upmanship then Back goes all in and risks the house.
Or, as the case is here, the pub. That's because the stakes at play in Back are the ownership of the John Barleycorn, a failing family pub in the English countryside.
In an easy example of the show's darker tone, Back begins immediately after the death of the John Barleycorn's beloved publican and frequent foster parent Laurie. Mitchell plays his biological son Stephen, a naturally despondent fellow, and Webb plays the charming Andrew, a likable and long-forgotten former foster child who lived with the family for a few months when they were lads.
Andrew's surprise return for Laurie's funeral is embraced by the family but treated with immediate distrust and suspicion by Stephen. Not least because his memories of Andrew living with them while growing up are hazy at best, non-existent at worst.
He quickly convinces himself that Andrew is a con man trying to smooth-talk his way to ownership of the family pub, while Andrew insists he's only come back because it was the one place in his eventful life where he felt truly happy.
The show offers no answer to Stephen's frequent question of, "why are you here?". Instead it muddies the waters with utterly contradictory flashbacks. Andrew will start reminiscing about a blissful sun-filled afternoon and we'll see young Stephen being rained on. You're never quite sure whose versions of events are true or if the events even happened at all.
As the episodes tick over the mystery deepens and behavioural extremes are inched closer towards. Andrew's conduct gets decidedly more shady and Stephen's desire to uncover the truth gets more and more fuelled by obsessive madness and increasing amounts of alcohol.
When his ex-wife mentions his whisky consumption he shrugs it off, saying, "I'm not guzzling. It's a single malt. It's studied appreciation," before guzzling down the rest of his glass.
The show's humour is pitch black, socially cynical and filled with merciless zings and devastating one-liners. There are scenes that are so awkward and situationally uncomfortable that they leave you squirming in your seat.
But it also has a real heart beneath the heartlessness. There's moments where it's incredibly moving without sliding into sentimentality of getting treacly.
Obviously, it's great. And while it's true that the pair are in their familiar roles - uptight square and cool cat - Back stretches their two archetypes like it's pulling back a slingshot that's aimed directly at the limits of audience acceptance.
Fortunately, the show's aim is true and it lands a direct hit on the ol' funny bone. Appropriate considering Back is often painfully funny.
