Just know your stuff...study the company, what they make, the FDA and quality processes they implement, especially study how the position you are applying for relates to your prior work experience and to Watson specifically.
Be confident...they are the type of company that will ask you those horrifying, "can you describe a time when....and how did you handle that situation" questions we ALL despise, so try to be cool and not get too nervous.
Definitely bring a couple of hard copies of your resume, just for good measure. Dress business professional..and it may not hurt to write down some questions to bring with you to ask them.
I worked at Watson from 2006 to 2008, just in case you were wondering, and started through Kelly Services staffing agency in Corona, CA..you could try going that route as well. Good Luck!!