Preparation saves time and money …
When hiring in a group of pro musicians, one of the real advantages is that they will usually be extremely accurate sight readers; meaning that if you put properly notated, written music in front of them, they bought to be able to look at the score and play it well straight away, without much practice or rehearsal. Therefore, it makes sense that the more work you do in score preparation before the session, the less of their time you will need to pay for. As an example, scores produced by midi are notoriously full of errors and inconsistencies which make it difficult to play, particularly at speed. Hiring a professional orchestrator, arranger or musician to look through the score beforehand and make any necessary adjustments can save many hours of paid studio time and mean that your session musicians will not need to spend the first hour correcting their own parts. This is particularly essential if you are hiring an orchestra or group of classical players (such as a string quartet) as any problems in the written part will need to be corrected by each individual musician as their paid session time ticks by. In a nutshell, well prepared music is going to keep expensive session time to a minimum.
Who is the composer?
Although many session musicians do improvise as well as play from written music, one area to definitely discuss beforehand is authorship. If you have part of a track where you'd like one of your studio musicians to improve a solo, it needs to be agreed who owns the copyright to that part of the track. If you have written all the music yourself and are simply asking a session musician to perform your work during a recording, then clearly you are the composer … but if the player has actually composed any part of the music themselves (even a few phrases that may have been improvised), then you may face a situation where you will need to assign a percentage of royalties to the player when the track is released and broadcast. There have been countless lengthy legal disputes over this very issue, so a clear discussion and contract with session musicians before they enter the studio will mean no hidden surprises later on.
What about arrangers?
Hiring an orchestrator (or another professional such as a specialist string arranger) is common practice for bands and composers who want to use orchestral instruments such as strings to their best effect. A trained arranger will know how to really bring out the qualities of strings in a track, use the range of the orchestra to add interest and harms within the song and will usually present you with perfectly rated sheet music ready to use straight away in the studio . If you are going to the trouble of hiring in good orchestral players, it makes sense to let an expert score the parts so that the end result will sound even more stunning. Again, if you are asking an arranger to write original material (such as a counter melody), then you should discuss whether they will do this on a 'work for hire' basis or would expect to receive royalties on the final track.
Where do I find good session musicians?
Recording engineers are ideal people to ask for recommendations as they will have a good idea of who is professional, accurate and easy to work with. These days, online collaboration means that it's possible to hire professionals from virtually anywhere, get your music recorded at a studio on the other side of the world and simply sent to you via the internet, ready for mixing. You can approach a few orchestras or groups to find competitive quotes (always ask to listen to samples of their playing to make sure you are happy with their level of performance). Similarly, finding an arranger or orchestrator online may mean that you work closely together on your track, despite never meeting in person.
Real or samples?
These days, the cost of high quality, orchestral samples have become much more affordable, meaning that composers and bands can produce music in the studio, using fairly realistic orchestral sounds … (brass and woodwind samples are still more obviously fake than strings and percussion) yet it is still possible to tell the difference and many producers and film composers always prefer to use the real thing whenever possible. One common practice is to simply hire a few excellent professional players to perform a layer of recorded live parts over the top of a bed of samples – this can add authenticity and sound far more convincing, without the expense of hiring in a full sized symphony orchestra … and the powerful studio to put them in!