The marauders were always really close, but James and Peter knew when to "back off" for a while to give their two friends some time together. James would make up some fake thing to do and drag Peter along.
Remus, being super duper smart and all, totally knew what they were up to (besides no good, Lol) but he'd pretend to believe their stories. Sirius didn't ever realize what they were doing and thought it was a great coincidence.
During these hours, the two mostly partook in make out sessions on the couch because Remus was such a good kisser in Sirius's opinion. Sometimes they would just snuggle in James's bed together. Either way, they spent those times just letting each other know how much they loved the other one, through actions and words alike.