During the "Shaving Lenny - Streets of hope" mission, eliminate the first two targets and then return to the door that was locked. Unlock the door, then in the new area walk straight until you reach a dog cage with two dogs in the middle of some very frisky business.
During the "End of the Road - Desert" mission you'll be alone with Lenny in the desert to deal with him in whatever manner you choose, but if you don't feel like getting your hands dirty there is another way to handle him. Look for five vultures throughout the level and shoot all of them down. Shortly afterward an unexpected visitor will come out of nowhere to help complete your task for you, but keep your distance if you don't want to become roadkill yourself!
The two playable characters from Kane & Lynch make appearances in this game, one during "Welcome to Hope - Great Balls of Fire" mission which is hard to miss; head into the bar to find Kane standing about. You can even drag him into a barfight if you want to get physical with the patrons here.
You can spot Kane again during the "Holding Cells - Skurky's Law" mission. Head into the sub-basement to the enter the vent. While you're making your way through the vent, peek through the grate.
Lynch can be found in the outdoor gun range during "Birdie's Gift - Gun Shop" mission.
Another cameo is less obvious to find. During the "Dexter Industries - Factory Compound" mission, you'll eventually be in the Enter Industries building. Look for a nuke hanging from the ceiling here and shoot its very tip. This will trigger the hidden "Atomic Suntan" Game Over cutscene featuring Kane and Lynch; be sure to save your progress before trying this!
During the "Rosewood - Orphanage Halls" mission, look throughout the level for figurines depicting characters from Mini Ninjas, another game that was produced by IO Interactive.
Shoot the items on the bench, next to the pool and on top of the bear in Sniper Challenge to get a stripper to appear in a window of the building.