In #53 The Answer, Tom's Yeerk brings what he thinks is a bludgeoned and beaten Cassie (not in morph) to Visser One. He then says:
One of the Animorphs, Visser. My people captured her and questioned her. Questioned her... forcefully.
Now, Tom's Yeerk is actually trying to trick Visser One. He hasn't actually done any questioning. However, presumably he would try to tell a believable story to the Visser.
So, why would Tom's Yeerk have to torture anyone? Why would the Yeerks have to question anyone to get information out of them? If they have a human they need information from, why not just stick a Yeerk in their head ASAP and get all the information? Why is Tom's Yeerk telling the Visser he tortured Cassie?
Obviously, if Cassie would have been in a morph too small to infest, it would have been another story. But here, she is in human form. (Actually, it's not even Cassie, but neither of the two Yeerks involved know that.)