Unfortunately, there is no guaranteed method to get good recordings in any situation. There are a though, a few considerations which you need to think about for any recording scenario - even when using a simple setup.
The main considerations for the example you give are:
- How much background noise is there?
- How loud is the person going to be?
- How will the table be used, and how do I mount the recorder on it?
As you vary the distance between the speaker and the recorder you are altering the balance between how much direct sound and how much reflected sound (from the room) you are recording. When you get the machine try this out and you will see what I mean. It really depends on how you want it to sound. If it's a quiet room, then the table will probably be ok. But if it's a noisy environment you might need to reduce that distance between the recorder and the person you are recording. Similarly, if they speak very quietly you might also not be picking up enough. The best way is to take some headphones, listen, look at the meters and make a judgement from those.
If you want to use the table then you will probably need a mini tripod of some kind, so you can point the recorder at the person. Also bear in mind that any bumps of the table might be picked up from the recorder (coffee cups etc). Isolating the recorder with a suspension will help with this, but they don't come cheap.
Also, do you want to record the voice of the person asking the questions? Then it gets a bit more complicated. I've done this before with the recorder in a vertical position between me and the person I'm interviewing, so one of the stereo mics is roughly pointed at each of us. It works ok, but is a bit risky, especially if they move around a bit. I'd probably only do it if the recording was for my reference only.
The other options are to buy two external mics and use them. Two short shotgun mics will give you more directionality, better sound and greater rejection of background noise. It's all just a question of what you need and how much money you have.
Best thing to do though is get the recorder and do a few test recordings. Try a few different environments, different setups, and see for yourself. The DR40 has xlr inputs, so if you need to expand the system with extra mics you have the option.
If anything doesn't make sense point it out and I'll elaborate further..
Best of luck,