I can accept that these characters have personalities, they need that. On the other I never saw any indication that either Fifield or Milburn were 'the best in their field'. There's wasted opportunities throughout their screen time to provide us with evidence - but it never happens.
Let's take the Geologist for example, because he's the easiest by a mile and more importantly, is presented with the earliest opportunity to demonstrate his professional capacity and validate his presence aboard the Prometheus. During the descent to the surface, one of the characters (I forget who... Holloway?) comments on the markings on the surface of the planet and words to the effect 'Nature does not work on straight lines'. How much more appropriate would it have been if the geologist made this remark? ...or at the very least, AGREED?
Gone is also any parallel we witness in 'ALIEN' where Ash analyses the surface using instrumentation and discusses the 'Rock lava base' composition of the planet they just landed on and its atmospheric conditions. Wouldn't it be great if Prometheus' geologist actually offered some information based on his professional experience in a similar way?
Then on the actual surface he does nothing. NOTHING. A BIG FAT NOTHING!!! He doesn't comment on the mountain ranges. He offers no advice on the integrity of the surface (for all we know, the surface could be soft, like pits of loose material and not safe to walk on). He makes no comment about the obvious roads not being natural - or asked for his opinion on them. He fails to pick up a single rock. There's so many excellent opportunities for a geologist to demonstrate his capacity for such specialised knowledge - which is why he was aboard the Prometheus in the first place - but none of them are taken.
I'm sorry, but if the character can't fulfill the expectations of his Profession - especially when he's reputed to be 'The Best in the Field' - then no amount of inane dialog such as "I love rocks" will help convince me he's adequately written as a character. If during the approach to LV-233, Fifield had f**k all to do except look out the window and talk shit to other crew members simply because he's a planetside specialist; but then his demeanor change to a more professional one on close approach to the planet's surface when his knowledge kicks in, then yes. That would be realistic. That's how professional people do behave. When the occasion arises, a Professional, in top of his game will put aside all other considerations and demonstrate his true worth. We see none of that happen in 'Prometheus'.
Unfortunately, Fifield's character as we have it is in the wrong movie. When a professional's dialog amounts to "I love rocks" and "I'm only in it for the money" then he would have been better placed in a movie like 'Idiocracy' rather than the symbolism laden background and existential investigations that 'Prometheus' attempts to deliver but is mired by inanities such as Fifield's character - and other stupid shit.
...Milburn is no different. But for the sake of keeping this post at a decent length, you can apply the same prinicpals to him as well. You'll find the results are more or less the same.