I understand that we are (generally) not allowed to ask a gentile to do m'lacha for a Jew on Shabas. Suppose he's going to do m'lacha anyway, but I want him to do it differently, or to do a different action instead, possibly entailing a different type of m'lacha, but not entailing more m'lacha actions or more m'lacha types. If he does my action, he won't also do his originally intended action. Is it permissible to ask him?
I don't have an explicit source for you, but there is a Ramma at the end of siman 244 concerning a non-Jew who is working for a Jew in an allowed situation (tax collector) where the Ramma warns that the Jew may not sit next to the non-Jew while he is performing this task.
The Mishna Berurah cites a Taz and other Achronim who quantify this prohibition and limit it to being involved in the actual dealings, whereas sitting there to protect the money from being stolen, that would be allowed. The final admonition there reads 'however be careful not to talk to him (the non-Jew) at all concerning the work.'
It seems any involvement with how the work is done would be a problem of mimtzo cheftzecha.
(For all Lubavitchers reading this, the footnote on the bottom brings that the Shulchan Aruch HaRav only allowed sitting there for hefsed meruba)