this depends on the job.
For example, if you run a shell script, AppleScript waits until the shell line has been finished.
To avoid this, you must save the shell script line as an external AppleScript application,
which will be called from the main script.
To toggle a button with Start and Stop you can use something like this
on clicked theObject
if title of theObject is "Start" then
set isRunning to true
set title of theObject to "Stop"
else if title of theObject is "Stop" then
set title of theObject to "Start"
set isRunning to false