In your startup, you should be looking for collaborators who challenge your thinking.
This does not mean they are 'assholes' or devil's advocate just for the sake of it, but rather they are sure to delineate the risks (so that they can be managed), help reveal blind spots, and fill in the blanks in terms of your own skillsets and capacity.
In some cases these people should make you a little uncomfortable because they're giving you new data and areas of focus that you hadn't previously understood or considered important.
It's important, though, that those people are able to 'disagree and commit'. Once you make the call, they should get on board and work their heart outs to make your decision and vision a reality.