The first thing to accept is that you won't have enough time, the location won't be easy, and your subject will either turn into a wallflower or a Prima Donna the moment you train a lens on them. But, if your interview is well scripted and your lighting is complimentary, you'll witness an almost magical transformation that can be quite revelatory.
You'll need at least two lights, preferably three. A soft source is best for your subject, a second light for the background, and a third for either fill light, hair light, or a side light (variously called "edge" light or "rim" light). While you can get away with bouncing your light from an umbrella for your main or key, umbrellas tend to scatter light all over the place, making subtlety difficult. A softbox will control spill, accept gels, and offer other accessory options. I have a love-hate relationship with grids: love to use them, hate to pay for them, but once you do, you'll know where the money went. They generally come in 20-, 40- and 60-degree angles. The smaller the number is, the narrower the beam will be. A grid will give you a soft, directional source—almost a contradiction when you consider the characteristics of most bare light sources.
I'd go for a 16 x 22" box (which fits almost anywhere) or a 24 x 32" with a 300-500 watt lamp. Starting position should be 45° off axis both horizontally and vertically to the camera (see diagram).
Used this way, you shouldn't need a fill, but if you wish you can use another soft source on axis with the camera and 1 to 2 stops weaker than your main light. I like 12 x 16" softboxes for this, and again, a grid is handy. If you elevate the light slightly, a 20° grid should drop the light beam behind the subject after providing fill, without hitting your background.
The background light should be a controllable, directional source with some focusing capability, and it should accept barndoors, gels, etc. Choose an area of the location that represents the interests of the subject. Offices usually have bookcases, for instance; an orchestra conductor's office may have a music stand with a score open on it, or a wall of the conductor's favorite antique batons. A gel will add some mood to the scene and make the mundane a bit more exotic. 250-300 watts should be enough for this. You want to bring the objects into view, but you don't want them to overwhelm the shot. Some specialty lights like the Dedolight feature optical projection attachments (sometimes called "cucaloris" or "cookies") so you can add Venetian blind-like or leafy break-up patterns to the background. Rosco makes hundreds of patterns to choose from. You can even project a custom slide. Gridding your main light is especially important here to maintain the integrity of your carefully lit background.
An alternate set up (see diagram) uses the 12 x 16"; softbox as a side light instead of a fill. Place the light at the subject's head-and-shoulder level and slightly behind, just glancing off the head, cheek and shoulder area. Again, a grid is handy in controlling spill into the lens. If you're using a raw light, use barndoors for this. You might want to gel this light too: orange sidelight and blue background light or vice versa; the warm-cool color friction usually makes for a compelling image.
Although fluorescent sources are increasingly popular, they're a bit bulky, and so, tungsten is still more widely used. This means you have to assess the color balance of your room. Are there windows and if so, what's the exposure? North or east will spread a bluer light than south or west, which will be warmer. Is the room lit with overhead fluorescents? What kind? Is it lit with high-hat recessed fixtures or MR-16 halogens? Can they be turned off?
In order to white balance, you're seeking fairly homogenous light conditions around your subject. But what if they're not? Balancing out one problem can frequently aggravate another. You could overwhelm the ambient light with sheer wattage but that would require a lot of weight and equipment. Interviews are all about speed and portability.
Let's assume for the moment that you are using tungsten light. Your lights are 3200K, and the room lights are either 2500-3200K, if they're tungsten. If they're fluorescent they're 3500K for warm white, 4500K for cool white or 6500K for daylight balanced.
Kelvin represents just the red-blue spectrum of color temperature. Most fluorescents have a degree of green to contend with: up to 30 points. Throw your window light into the mix and you have three disparate light sources which you can't white-balance away. A large film production crew would gel the windows and overheads or sleeve the fluorescents to match your lights, but I'm guessing that's not in your budget, either time-wise or financially.
Plan B is to just turn the overheads off or at least disable the lamps in the immediate area around your set. Then you've only got the windows to deal with. Mid- to late-afternoon sun will probably come in close to 3200K, but I'd have some sheets of Rosco 3316 (1/8th blue) or 3208 (1/4 blue) on hand to put on your lights, just in case. For strong blue north window light, pack some 3204 (1/2 blue) or 3202 (full blue) which will raise the color temperatures of your lights from 3200K to 4100K and 5500K respectively, to match the incoming window light. Then you can read for white balance. It's not written in stone. It just has to look good. Of course, you can eliminate this entire step by shooting at night, or in a windowless area.
I haven't listed filtration to match the various temperatures of overhead fluorescents because I'd rather exhaust every other possibility first, but you'll probably be adding #3304 (Rosco "Plus green"; approx. 30 points of green) for 3200K lamps, #3304 + #3204 for 4100K lamps and #3304 + #3202 for 5500K lamps–if you go this route.
It's a bit confusing, but the concept isn't: make all your light sources the same color before white balancing. Remember, these gels are added to your light sources to match them to the ambient source.
The above is just a guideline to which to apply your personal style. There are also exciting new products, especially in the field of LED Sources and HMI's, for those with larger budgets. Consult our website for details, or feel free to contact us by telephone and speak with a lighting specialist.