For kind-of, sort-of realistic modern-day people who won't utterly crumble in adventure situations,
GURPS Action 4 mostly just needs a half-priced BAT:
Attributes: ST 10 [0]; DX 11 [20]; IQ 11 [20]; HT 11 [10].
Secondary Characteristics: Damage 1d-2/1d; BL 20 lbs.; HP 10 [0]; Will 11 [0]; Per 11 [0]; FP 11 [0]; Basic Speed 6.00 [10]; Basic Move 6 [0].
Advantages: 20 points of advantages chosen from Luck [15] or the big list on p. 5.
Disadvantages: A total of -40 points in disadvantages picked from the many suggestions on pp. 5-7.
Skills: Brawling (E) DX [1]-11; Computer Operation (E) IQ [1]-11; Driving (Automobile, Heavy Wheeled, or Motorcycle) (A) DX-1 [1]-10; Guns (Pistol) (E) DX [1]-11; Stealth (A) DX-1 [1]-10; and Wrestling (A) DX-1 [1]-10.
Slush Fund: 4 points.
Use this with the process flow on p. 4, but limit yourself to one skill set in a 75-point campaign or two in a 100-point campaign.
For instance, you might build a plausible fireman who just happens to know how to fight and shoot by starting from the above BAT and choosing ST +1, Will +1 (to be brave!), and Fit with your advantage points; picking your disadvantages from the Dutiful and Virtuous lists (because you're a life-saving hero); doubling up on the Fireman skill set with the notes on realistic campaigns in effect; using the Slush Fund to buy some realistic background skills like Area Knowledge (to know the city) and Urban Survival (to be able to spot dangerous structures, standpipes, etc.); tacking on a Higher Purpose as explained on p. 34; and optimizing.
ST 11 [10]; DX 11 [20]; IQ 11 [20]; HT 11 [10].
Damage 1d-1/1d+1; BL 34 lbs.; HP 11 [0]; Will 12 [5]; Per 11 [0]; FP 11 [0]; Basic Speed 6.00 [10]; Basic Move 6 [0].
Advantages: Breath Holding 2 [4]; Fit [5]; Higher Purpose (Keep this city standing!) [5]; Lifting ST 2 [6].
Perks: Special Exercises (Lifting ST 2) [2].
Disadvantages: Duty (Fire department; 12 or less) [-10]; Overconfidence (12) [-5]; Pacifism (Cannot Harm Innocents) [-10]; Selfless (12) [-5]; Sense of Duty (Brigade) [-5]; Workaholic [-5].
Quirks: Whatever you like! [-5]
Skills: Area Knowledge (City) (E) IQ+1 [2]-12; Brawling (E) DX [1]-11; Carousing (E) HT [1]-11; Climbing (A) DX+1 [4]-12; Computer Operation (E) IQ [1]-11; Driving (Heavy Wheeled) (A) DX+1 [4]-12; First Aid (E) IQ+2 [4]-13; Forced Entry (E) DX+2 [4]-13; Guns (Pistol) (E) DX [1]-11; Liquid Projector (Water Cannon) (E) DX+1 [2]-12; NBC Suit (A) DX+1 [4]-12; Professional Skill (Firefighter) (A) IQ+4 [16]-15; Stealth (A) DX-1 [1]-10; Urban Survival (A) Per [2]-11; Wrestling (A) DX-1 [1]-10.
The GM might be sympathetic to lowering the Professional Skill to boost "action" skills if the campaign is mostly going to be about that (say, to get Brawling (E) DX+1 [2]-12, Guns (Pistol) (E) DX+1 [2]-12, Professional Skill (Firefighter) (A) IQ+2 [8]-13, Stealth (A) DX+1 [4]-12, and Wrestling (A) DX+1 [4]-12 . . . though if the game will have our fireman suddenly thrust into an action role, it won't take many earned points beef him up), but this is a playable and fairly competent 100-point character.