I recommend not doing this unless absolutely necessary. Remember a trigger program is called whenever an update, add, or a delete occurs on a file based on the event you select and this means interactively and in batch jobs. In our company we had such a program that would lock jobs until a user answered the display. It proved safer to call the program to display the screen in the actual program that caused the update and removed it from the trigger program.
If you do decide to proceed with this make sure it is programmed correctly and you do not try to open the display file in a batch job or if will abnormally end and that could cause more harm than good. Also make sure locking a record within this does not cause a problem in other jobs trying to access the data.