I started out by just trying to add conflict to the gist and then went back and fleshed it out a bit. I'll give both stages.
I started with this:
Having a sudden idea
Tim begins to have ideas about others. He’s starting to see them all as monsters, as aliens that are all conducting experiments on him. Having these ideas leaves him battling against inner turmoil, feelings of guilt, disbelief, etc.
Pretending to be blind
Tim pretends that he is unable to see the monsters under others skins. His difficulty with pretending to be blind draws the monsters attention.
Planning to escape
Planning to escape from the monsters leaves Tim conflicted about leaving his wife and daughter. He’s sure that at least his daughter is human and on his side. When he tells his plans to his daughter, she tells the others what he’s doing.
Being changed by someone else
The aliens capture Tim and reprogram him, resetting and restarting the experiment. He’s terrified at the idea of the experiment continuing.
I fleshed it out, only a bit, to this:
Tim has a normal, boring, repetitive life. One day, he begins to have these sudden ideas that he’s not alone, that he’s being watched. The ideas themselves are easy enough to dismiss, but he can’t stop them from popping into his head and distracting him. Others start noticing that he isn't acting right and start telling him that everything is fine. But something just seems off about everything. He suddenly has the idea that everyone is an alien except for him, and they are all performing some kind of experiment that involves observing him. He tells himself he's crazy, but the ideas keep coming and making him crazier.
Tim has come to accept the idea that he is being experimented on. In fact, now it seems rather obvious what’s happening. He can even see the monsters under the skin of his friends and family, or what he thought were his friends and family. He thinks even his wife is could be one of the aliens. His daughter is the only other one he thinks might be human, but he can't tell her. The more he tries to pretend not to notice the monsters, the more the monsters seem to notice him acting funny. They see him growing more nervous, his palms beginning to sweat. They see they way he flinches and looks away from others, and the aliens are beginning to suspect something.
Tim has decided he has to get away, to find a way out of this experiment. He doesn’t want to leave his wife behind, but he thinks she might be one of them. Not his daughter, though. He’s sure she isn’t one of them. He makes plans to get out, but there’s no way to leave and be able to take his daughter with him without telling her something. So he tells her whatever is required, that she needs to pack or something, and starts looking for his chance to escape. But when his daughter, scared by her fathers actions, tells her mom what he said the authorities are called and they come to get Tim before he can escape.
When Tim sees the doctor--a short, thin man with a roundish head and large, round wire-rimmed glasses over beady eyes--enter the room, Tim knows what’s really happening. That man isn’t a doctor, he’s an alien. And he’s come not to kill Tim. If that was what was happening, they’d have done it already. Something quick, surgical, clinical. No, there’s too much going on for that. They’ve come to reprogram Tim. To reset his mind. So they can restart the experiment, maybe. Tim breaks down. He can’t stand the thought of starting over, going back to being completely unaware that the aliens are there, back to living with his alien wife, his alien daughter. The doctor lifts a needle, sticks it under Tim’s skin, and depresses the plunger. A moment later all goes dark.