Sirius wasn't lying when he said he had plans, big, huge, epic plans. Plans that would be marveled at for centuries! Alright, well, maybe not that last bit but they were really clever, his plans, and James clearly had to come and tell him as such. After all, Sirius rarely flew solo on any of these such things. Nope, it was him and James. They were like Peanut butter and jelly. They just went together and clearly whenever one of them had something absolutely brilliant flitting about in their heads they had to share it with the other. Which was precisely why he demanded James come to the kitchens right NOW so he could relay these epic and genius plans of his to him. This way they could begin planning and hopefully successfully execute said plans.
Still, waiting for James to arrive was so boring and Sirius hated being bored. He had tried everything, he'd whistled, he'd swung his feet noisily, he even struck up conversation with one of the house elves to no avail and eventually just ordered his own lunch and dug in instead of waiting. He was half way through his own meal when James finally did arrive, grinning at him through a mouthful of sandwich as he sat down.
"I'm important in and of myself!" Sirius said, feigning hurt as he swallowed what he'd been chewing.