Formative Usability Testing – Script

I’m running my first real usability test. I am doing this for an internal, enterprise application that will eventually be offered as a SasS.

I’ve recruited a small user group which literally account for 100% of our national users. (It will be an international product for a very niche industry.)

I’ve done a task analysis and have had multiple opportunities for contextual observation.

I’ve analyzed their current system, interviewed current users and reviewed analytics and business data to better understand user and business needs.

I’ve created a medium fidelity, semi-interactive prototype to run some initial, formative usability test.

My question is: how important is the script?

I’ve created a small script of tasks. I.e)

1)Login, search for Customer "X" by Name.  2)Identify customer's mailing address.  3)Find navigation instruction to customer's address. 

Although these tasks are based off of a true representation of day-to-day tasks a user does, my understanding is that a Usability Test follows much more of a scripted version.

Is there anything wrong with doing a task based script if it is a true representation of a users day-to-day workflow?

Do I really needs to say things like “You are working for XXXX, a customer phones in. You need to locate this customer in the system.” Or can I just give the user very small, specific tasks.

I’m asking because this, being my first time, makes me uncomfortable. I would rather give them simple tasks and analysis. But at the same time, if this will skew my results, I’ll take the proper time to write a script.

What do you think? What has your experience been.