"Would" vs. "Will"
Your first example is correct most of the time. When speaking about something in the future, you will be doing it. When you've made plans to do something, and you are informing another person this is the form you'd use.
The second example is usable in some situations. You're dealing here with the "future in the past" tense, if I understand it correctly. This tense is often used when presented with new information that your prior plans may influence. We'll pretend that in your example, you are speaking to someone who has asked you to write a new script. You had already planned to write a different script.
From the link:
Future in the Past is used to express the idea that in the past you thought something would happen in the future. It does not matter if you are correct or not.
When you say "Please note that I would be working on another script this month", you're indicating that you already had plans to work on another script before the conversation. In general you would use this active-voice phrasing to let the addressee know that you still intend to work on said script, either instead of the new assignment or in conjunction with it.
If you were to use the passive voice ("Please note that I would have been working on another script this month") you would usually be indicating that while you had planned to work on a new script, you will no longer be doing so.
Both would and will can be used in reference to a proposed future event. When the speaker is the agent of the future event, as in this case, will can be used both conditionally, to indicate that the event may or may not happen, and unconditionally, to indicate that the event is definitely going to happen:
I will be working on another script later this month.
I will work on another script later this month, if I have the time.
By contrast, would is typically only used conditionally, and usually implies that the speaker does not think the event will take place, or is flatly asserting that it will not:
I would work on another script later this month, if I thought I could find the time.
I would buy the tickets right now, if I had the money.
(Would is also sometimes used without the conditional to indicate a desire or wish—"I would speak with you, milord"—but this usage is archaic.)
To illustrate the difference, here's one way you could beg off politely:
I would like to help you with your project, but I will be working on another script this month, and so I will not have the time.
Either seems correct to me, in the given context. However, the connotations are slightly different. Will could imply that you plan to work on the new project, but won't be able to devote all of your time to it, whereas would might imply that you could work on the new project if absolutely necessary, but you'd really rather not (because of the other script).
- The use of will seems like a more definite answer as no matter what happens with the current script you will work on another one
- The use of 'would' is a more of a conditional where will work on another script depending on what happens with the current script