Grumpy Sims may scream out in fury if something bad happens to a nearby Sim or if something bad happens to themselves.
Grumpy Sims will autonomously use the "Complain About..." type interactions. This will grant them the positive "Let off Steam" moodlet if the interaction is accepted.
Sims with the Grumpy trait may develop the "Out of Sorts" moodlet upon awakening, making them potentially angry.
Grumpy Sims will not react well to being splashed in a pool, and will furiously splash back at the offending Sim, decreasing the offending Sim's fun gain. They share this attribute with hot-headed and mean spirited Sims.
Grumpy Sims will autonomously "Shoo Off" cats sitting on top of furniture, even if the cat is asleep.
Out Of Sorts
Occasionally appears on grumpy Sims upon waking up. The "Out of Sorts" moodlet adds a -20 point penalty to the Sim's mood, and it typically lasts for 23 hours. Getting a good night’s rest or meditating[TS3:WA] can help to alleviate this moodlet much faster. Neurotic Sims can also remove this moodlet through the use of the "Share Worries" interaction.