A couple of people in my group were fairly new to the 40k setting and had no real life experience dealing with military/paramilitary bonds. As such they were having a hard time figuring out and acting out how their characters would really feel being dropped into the middle of a DW KT, especially given the oaths and whole professionalism thing that Space Marines tout about.
One suggestion that seemed to resonate with them was with their own previous characters in other RPGs and campaigns- as Deathwatch is a 'high level' campaign in comparison to some of the other lines I suggested this to them: Treat your Deathwatch Marine the same way you would treat a high level DH, RT (or WoD, or D&D) character who had been fighting along side a set party for a whole campaign. Then take that character and plop them in the middle of a new group of characters. How do they interact with people they've never met who claim to be elite, but you've not seen true proof of this, versus the people you essentially grew up with, who you trust like no other. Somehow this related better to them than some of the other descriptions given- if you have players that struggle to adapt, this may be something they can relate to better and help them find their 'motivation' so to speak.