Isolating one character from the party lets you play to their weaknesses. As the party is strong and balance each other out, a lone character has fewer options.
It is also an opportunity to put an unwanted spotlight on a character.
Looking to Hollywood for inspiration there are three versions of this.
- Looking for the burgular. We all know that the person who hears a noise so goes to investigate on their own is in for trouble. This one works well in D&D, which is unusual as not a lot of horror stuff works for games heavily focused on combat. The key here is to interupt long or short rests. The person on watch hears or sees something. They have three options, ignore it at their peril, investigate on their own, or wake the party. If the party gets woken and you can keep them active for an hour or under stress, they don’t get a long rest, and none of the benefits. They are not going to be impressed with that after a few such incidents.
If the watcher ignores what they spotted, you have carte blanche to throw a well organised attack at them. They had plenty of warning and failed to heed it. The rest of the party will not be impressed if the watch did not warn them of an impending attack.
The third option is to go and investigate what they saw or heard. Then you have them! - The gruesome discovery. The charcter goes off on their own and finds something horrible, scary. The “We need a bigger boat!” scene from Jaws is this kind of isolation event. These can also work well with things that can come and go. A character finds a body, goes and finds the party, when they return the body has gone. These isolation events explode the number of unanswered questions.
- Private communications. Something tries to contact the character that is on their own. A glimpse of a different face in a mirror, a token left on a bed. These isolation events often violate a characters private space. Maybe they are being observed or their bedroom is being invaded. The ‘other’ can reach the character when and where ever they are.
This is a powerful tool.
I suggest coming up with a list of potential isolation ideas, and then waiting for a player to mention something that they are doing that would mean that they are on their own. These normally come from the most casual of comments. Going to make a sandwich or going to check on their horses. As soon as you hear that, you take one of your ideas and improvise a quick scene around it.
If you plan too much, the ideal opportunity may never happen.