I can't really assert that I'm either crazy or not, I tend to walk the blurred line between the two; I don't have any diagnosed psychiatric issues, but I haven't exactly sought out diagnoses either, and what cognitive discrepancies I do experience aren't personally distressing enough to motivate me to do so. That said, I work around the issue of being caught talking to myself (I used to do that even before I had tulpa residing in my head; embarrassed myself a few too many times by being noticed doing it, so I broke the habit), by having writing be my primary method of forcing. That's where my tulpa all started, so that's where I continue working with them for the most part. We engage in exercises in visualization and such from time to time (probaby less than we should, but I have other rsposibilities to prioritize), and hold conversations, but those conversations stay silent to the outside world and are held in my mind. It works just as well as if I were speaking to them aloud, because I know they can hear me from inside anyway, so it makes little difference whether I speak it or think it. However, writing is (or at least was) still the medium through which I gave them the most creative energy that they feed on.
I worry on regular enough basis about being perceived as crazy, just not from working with tulpa.