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Mishk-
I'm trying to create a A0 cinema poster from an HD video
A0 is 1189mm high in portrait orientation. HD video is 1080 pixels high. Do the math, as they say: 1080px spread over 1189mm is 23dpi. Bear in mind also that a screen grab from video may be shite to varying degrees compared with a 1920x1080 still image, depending on how lossy the video codec is and exactly which frame you grab.
This is what happens when you resample a 1920x1080 image to the resolution required to print the image 1189mm high at 300dpi
gbj_tester
If you mean print at 300 pixels per inch (dpi isn't the same thing at least in the print world) then that's simple in e.g. Photoshop, but if the image is 1080 pixels across, then the print will be 3.6" across.
If you want to print the existing image bigger, it won't be at 300 ppi.
If you want it bigger and 300 ppi, then you need to resample (i.e. interpolate) to add more pixels. Whether this makes any visible difference is very dependent on your output process, paper, etc.
Remember "300 ppi for print" is a rule of thumb for photographic images on high quality paper viewed close up. It doesn't apply for large posters, banners, etc.
What are you doing with the print?