Yeah, it's all a persona/mask with them for the most part.
Aside from a few. I used to know Stevie Nicol's niece, quite well shall we say, and I met him on numerous occasions when he wasn't sunning it up. He's as grumpy in real life as he is on camera, but maintains that Scottish sarcasm and he'll often wink at you after what seems like an insult. The whole Mr. Grump thing isn't an act though, that's him. His wife is lovely however, proper welcoming.
I met Grobelaar once at a charity do. He's as mad as a box of frogs and very salty, or at least he was then as he'd clearly sunk a gallon or so. I can't imagine him ever being a pundit, or if he was they'd never let him loose on a live broadcast.
Worst one was Teddy Sheringham, another charity do with a lot of really rich people bidding on memorabilia, again for charity. What an absolute clown that man was. If I'd have been 20 when I met him I'd likely have snapped his jaw clean in two, but I was about 35 and by that point had learned to just walk away from Juan Kerrs as it's not worth the aggro. I genuinely did want to give a good right hook though, such an arrogant, rude and entitled see you next Tuesday.