essence versus style
I think, from your question, that you are confusing the function of planets and signs when combined. Dad posted a fine explanation, but I'll take another whack because we all learn differently.
Planets are our basic units of energy in life. They are what we need and do and we all have the same basic needs. Mars is the energy of defending ourselves, getting what we want (as dad says), agression, exercise yadda yadda.
The sign where Mars lands is How or in What Style we go about satisfying this need. Mars in Virgo isn't so likely to strut his stuff, but he could be the mild mannered guy you find out has a black belt in martial arts, which takes time and discipline as a craft (Virgo). If the guy with Mars in Virgo is an exec, he'll expect those around him to organized and on time for meetings. Mars, after all, imposes himself on others.
Mercury is the need/urge to communicate, think, etc. Leo provides an expansive or warm style to mercury's communication producing perhaps a blowhard or perhaps a colorful persuader. Never mind the details, though. If a writer had this combo, he'd be very likely to identify with his hero/book or to write an autobiography. And I'd expect it to be an entertaining book. A writer with mercury in Saturn, on the other hand could produce an incredibly well researched tome that puts everyone to sleep. But you'd get a lot of stuff for your money.
In one of my books (forgot which) the author describes a demonstration of mars in action for his class of students. He opened a big box of candy and invited them to take what they wanted. Some strode right up and plucked a piece looking neither right nor left (mars in Leo maybe?), others smiled, sidled around politely, inspected the candy carefully before selecting a piece (in Virgo or Libra). Some stayed by the box and took a couple of pieces (in Taurus?). Everyone got their candy (WHAT they wanted) but they exhibited this need differently (how/style).
You can make up some stories for yourself along these lines. Take an function of a planet (say the exercise part of mars' energy), then add the how (sign) and house (where) for your character. Examples:
1. Mars in Geminii in the 4th house...this guy has a lot of equipment at home (4th house), only some of which he uses and even more video tapes. He switches from one piece of equipment to the other and watches his tapes while talking on the phone (Geminii). He does a lot of reps with light weights.
2. Mars in Sag in the 3rd house ... he's always seen at the (third house) gym (why stay at home when you can show off those pecs?). Not content to talk on the phone, he's telling everyone at the gym his stats, progress, etc (sag). He huffs and puffs a lot while lifting big weights. He may change gyms if there is a Mars in taurus guy with bigger pecs.
Bibi x