gistfile1.applescript

-- 1. TextEdit got an error: Can’t get POSIX file "..."
set p to "/path/to/file.txt"
 
tell application "TextEdit"
  set q to POSIX file p
end tell
 
-- 2. This works
tell application "TextEdit"
  set q to POSIX file "/path/to/file.txt"
end tell
 
-- 3. This works
set p to "/path/to/file.txt"
 
tell application "TextEdit"
  set q to (p as POSIX file)
end tell
 
-- 4. This works
set p to "/path/to/file.txt"
set q to POSIX file p

posix file belongs to standard additions. When possible, don't include it in a tell block.

set p to "/path/to/file.txt"
set q to POSIX file p
tell application "TextEdit" to open q

Yeah I got the lesson out of this - the thing that drives me nuts is that it sometimes works and sometimes doesn't!