Continuously monitor files opened/accessed by a process

Try with strace -p 12345; it should do what you are trying to achieve.

The output can be filtered to only display opened files (Dan D.'s comment):

strace -e open -p 12345

Note: you can also trace quickly running processes with strace -e open <command>.

The new utility fatrace will do this: https://launchpad.net/fatrace/

sudo fatrace | grep '(6514)'

Don't use the -p option, it means the opposite of what it means in lsof or other utilities.

This will loop re-running your command and clearing the screen each time:

watch "lsof -p 12345"

WARNING: this will miss quick file accesses and is only suitable to see long standing files