I think I understand the attitudes of Space Marines and Tech-Priests. But a techmarine is a bit of both, so how do those mindsets come together ?

You just answered your own question. Techmarines are marines first and members of the mechanicum second.

Much of the fiction depicts Techmarines as cold or distant, but highly devoted to their Chapter and to the Emperor. Unlike the Mechanicus, a Techmarine's first loyalty is to the Emperor, and and they use the teachings of the Mechanicus as a means to an end, not an end in and of itself. Any of the following traits (or a combination) could be a good starting point for roleplaying your Techmarine:

• Serious
• Meticulous
• Cold
• Focused
• Obsessive
• Mystical
• Calm
• Methodical
• Implacable
• Analytical
• Imperturbable

Techmarines (and the Mechanicus for that matter) are still human, and still feel emotions, but they express themselves differently. My suggestion would be not to use these traits to define the character, but as a lens through which his personality emerges. A silent, emotionless Techmarine is boring to play and boring company; but expressing anger, elation, humour, or sorrow through these filters offers a challenge and gives the character depth.

I would think tech-marines feel a little issolated from the rest of the chapter as well - they have that Mechanicus / Marine duality where they probably don't fit really well in either. It also offers you some fun role playing.

GM: The melta-bomb goes off, searing Brother Ferdinan's left arm to the bone and possibly fusing the track on the Land Raider Behind him.

Tech-Marine: "Have no fear! In the Emperor's name I will save you!" runs past the Marine and ministers to the Land Raider

Aren't Techmarines covered in the core book? I've only had an hour or so with the book (okay a little bit more after last night), but I was looking at some other stuff and not the Techmarine material. Anyway, if they're in there surely there should be some information on how to RP them?

Kage

Hopefully for the techmarine, the teaching he redieved tell him that the Omnissiah ( the avatr of the machine god) is the Emperor. Allowing him to not becoming mad and to know like all his other brother a total faith and loyalty to the Emperor.

Tha fact is that he would certainely kill every members of the adeptus mechanicus telling him something else. He had been first raised as a SM.

Darq said: Published on 10/2/2010 - 10:19:50

I would think tech-marines feel a little issolated from the rest of the chapter as well - they have that Mechanicus / Marine duality where they probably don't fit really well in either. It also offers you some fun role playing.

GM: The melta-bomb goes off, searing Brother Ferdinan's left arm to the bone and possibly fusing the track on the Land Raider Behind him.

Tech-Marine: "Have no fear! In the Emperor's name I will save you!" runs past the Marine and ministers to the Land Raider

Maybe in abattle he must choose to help the Landraider and not his brother because he isn't the medic on the field of battle but it is his duty to make a powerfull weapon of the Emperor as a Land Raider to be "alive" again. If someone can do something for his wounded brother he is not the man.

And as Direach said they are still human and got feelings, maybe not those expected by a 100% human being, but feeelings none the less.