Alyson Hannigan kissed Jason Segel a lot while making 'How I Met Your Mother,' but not very much in their early days on set together. Hannigan doesn't have a problem with kissing, or with Segel, but she didn't like the fact that he was a smoker. Segel apparently wanted to give up, so Hannigan thought she'd help him by refusing to kiss him until he'd done so.
It wasn't just the filming impracticalities that acted as an incentive for Segel; he also agreed to pay Hannigan $10 every time he went out for a cigarette, in the hope that he'd eventually get sick of losing his money, and quit for good! With Hannigan's (good-natured) assistance - by which we mean she was on his back all the time - he eventually succeeded. From that point on, their characters were much more romantic together on screen.