Lister tells Rimmer he won't shoot him for finding him in bed with Kochanski as Cassandra predicted.
However, Lister was not present during the conversation between Cassandra and Rimmer where she told him he would be shot by Lister for finding him in bed with Kochanski.
Therefore, there is no way Lister could have known he was expected to find Rimmer and Kochanski in bed and consequently shoot him.
After the "canary" yells up to the captain, "Let's kill something," he briefly looks at the camera.