Fields and/or custom fields hidden from certain watchers or users

Hi, I sometimes share a task with collaborators or clients and it would be useful to be able to choose which fields can be excluded from a watcher's or user's view.

To give a quick example, I could open a custom field to record the number of hours a particular task took, this information would be useful to me, but I wouldn't want the client to be able to see it, at least until I decide they can see it (maybe at the end of the month… or maybe never)