James was very effective at taking people by surprise with his flailing, but near-blind grappling required more coordination than he had at the moment. Thankfully, between the alcohol in Sirius's face and his own state of inebriation, they were fairly evenly matched. And Merlin, it was just like being in school again, goofing off and wrestling in the shack. He couldn't stop laughing either, probably wouldn't have been able to even if Sirius hadn't been tickling him, but as it was he could hardly breathe.
He made a wild grab for something to use against Sirius, and ended up with a handful of stuffing from the couch; he shoved it in Sirius's face and rolled off him, cackling. He was certain Sirius would be after him in a second, but he used the respite to try to catch his breath. Ineffectively, because he was laughing so hard that all he could do was lie on his back and hold his aching stomach with his hands, knees pulled up to his chest as he shook with mirth.