Different interpreting projects may apply different criteria or additional criteria.The standards expected of an interpreter's ability has come under much closer scrutiny in recent years. The most basic being an absolute command of the English Language, in vocabulary, grammar and diction.  Likewise the language you offer as an interpreter will be subject to a similar scrutiny. Do not even think of applying for a post as an interpreter if you cannot meet these basics.  

They may:

Test your fluency in English (and score)

Test your fluency in the language offered (and score)

Look at your qualifications in interpreting (For example the DPSI)

Look at your qualifications in languages

Look at your all-round qualifications and educational background.

Look for the disciplines and experience offered: i.e. health, law, local authorities, social services etc.

Expect an In House testing by an assessor of your language ability and accuracy in a role play situation

Seek  proof of your  experience

Seek your willingness to undergo a Criminal Records Check

Evaluate your availability

Seek your willingness to abide by a Code of Conduct and Confidentiality which includes signing to, and  understanding of, such a code.   

EXPECTATIONS
 
Average payment for services and travel. 

 Although this does vary  and some interpreters will try and charge more for rarer languages, travelling time is distinctly very variable.
 
The average pay is between £15 and £35 per hour once the assignment begins. Usually there is a minimum payment from one to two hours
 
Travel to and from the assignment is less clear but this can be, nothing, or nominal to around £15 per hour.  With any fares or mileage allowance agreed on top. 

Most interpreters are self employed  (You have to register as such with the appropriate Government department)  Very few manage to make a decent living

Because the number of assignments is unpredictable, interpreters must market themselves, produce a good CV with referees, and get on to as many lists as possible, ie police, courts services, hospitals, local authorities etc. Making your own website is worth considering, even getting your own domain name is not very expensive,  just a few pounds a year.   

THESE NOTES ARE GUIDE LINES, AND ARE NOT 'SET IN STONE' . THEY ARE DESIGNED SIMPLY TO GIVE INTERPRETERS SOME HINTS ON HOW TO PRESENT THEMSELVES AND WHAT THE INDUSTRY NOW EXPECTS OF INTERPRETERS.