Probably lots of mentioning of shifting eyes, right? Weird mouth expressions / smiles not reaching eyes?
How do I sow seeds of doubt in reader's trust of one character while keeping my protagonist oblivious?
Change small behaviours. Distancing, overly kind, changing speech patterns. Lots of excuses time and time again. Small things, body language. I don’t know much, but that’s what I’ll offer
Have them lie or deny doing something for no good reason, where it doesn’t seem like a big deal, but they keep doing it as if they’re just THAT used to lying about things. Or have the character claim to be a fan of something or have a hobby or expertise in a field, but they’ll change the subject or avoid giving details when asked friendly questions about it.
Make them do something the average reader would dislike, but make it a flaw that your protagonist wouldn’t notice. For example, perhaps they do something bad, but the protagonist sees them as their friend, so they don’t immediately doubt them. Keep doing these little things until the big moment!
If possible, you can always try switching to another point of view.
But if that’s not possible, having other characters be suspicious of your main character over little things while your main character defends the person of interest would work splendidly, too.
Well, pretty much all my readers doubt a character that both the protag and I have no problems with
Apparently in their thinking, calling up someone who almost died to see if they are okay is ‘stalking’ and ‘not to be trusted’ * facepalm *
So, to answer your question, have the character do nice things for a stranger and I’m sure you’ll have everyone questioning their motives.