Lets say you have a formfield dropdown that you have bookmarked DD1 (to
keep it simple)

The possible values are Tea, Coffee, Beer. When you set this field up
check "Calculate on exit"

Now postition your cursor where you want the self populating field.
Don't put a formfield here instead use a ganged field construction
like.
Note all { } pairs are entered with CTRL+F9.
{IF{DD1}="Tea""Two Sugars"}{IF{DD1}="Coffee""Black and bitter
please"}{IF{DD1}="Beer""Why yes, I will have two, Thankyou"}