I got 2 weeks to learn 7 finger-burning songs on bass. Any ideas on
what should my strategy be?
Tough challenge based on how you're framing it up: Their songs are way more complex than pretty much everything I've played before.. I got 2 weeks to learn 7 finger-burning songs on bass.
First thing to do, if you are comfortable doing so, is to talk to your friend and ask what's expected of you. To play everything exactly as it's been played before? To fill in as best you can and just keep the groove going? Etc. That will give you an idea of how to approach your task.
Having said that, your situation is not at all uncommon. Working musicians are constantly faced with such scenarios, often far more difficult than yours, although such difficulty is all relative to your particular level of skill: A union musician gets a call - Be at the Hilton's bar on 6th Ave tomorrow night at 7. There's a trio playing there - they work until 11:30. The bass player is sick. I'm emailing you their set lists. You can find most of the music online. End of call.
Guy looks at the set list - 50 possible songs depending on what they feel like playing and how long they stretch things, and not including audience requests. He knows half of the material cold, the rest is vaguely familiar or unknown. He looks over the music or listens to it- some of it is fairly easy, some of it is very tough. So, one day to get down 25 songs - some work to do. One of the ways musicians often deal with this is called faking it:
It amounts to this: You don't necessarily try to play that finger-burning part. It's unlikely you'll be able to do it well, and when you mess it up, it's going to sound bad. Instead, you play something that sounds similar - produces a similar sound and effect, functionally adequate - but is well within your ability to learn quickly and play well enough without struggle.
Every tough, nuanced bass line has certain essential features, and other components which are embellishments and enhancements. You might find a bassline that's very funky, but its basic outline is simple. A hot-shot player might be throwing in 16th or 32nd notes or harmonics or double stops or adding chromatic passing notes or ghost notes as he blisters through it.
That sort of playing might intimidate you, and if you try to nail it all in a short time, under pressure, you'll in danger of failing: Sounding bad, wasting valuable time that could have been better spent with a different approach, maybe even losing your chance to gig again with some musicians you'd like to work with. But those parts are generally non-essential - the bassline and the song will work without them, as long as you can play the essential fundamental line, clearly and cleanly.
So your job is to listen to those bass parts carefully in the context of the song and pick out the dominant features that make it work and focus only those, ignoring the enhancements and embellishments. Play a simplified version that you know you can execute well. Very often, you can get by with a highly simplified bass part as long as it maintains the same basic groove and feel. Even properly placed and played roots and 5ths can cover for a great deal.
The key is to understand the original player's approach to the material, digest it and then, best done while playing along with a recording, develop your own simplified version that's within easy reach for you, but captures the fundamental feel and function of the original.
Don't worry about the fact that you're not playing the same thing as the original. Just keep in mind the fundamental role of the bass in that song and fulfill it, as best you can with your own abilities and the limited amount of time you have.
Whatever you play, make sure you can play it well: Good solid execution, a clean, clear sound, a strong, solid groove. Unless you're a star 'lead bassist', that's more important than the "finger blistering" stuff.
I remember a long time ago when I had to learn to play the Beatles' Come Together. It's a very simple song, but McCartney plays a cool bassline that jumps up to the b3 (if I remember correctly) on a higher octave and then quickly and smoothly slides back to the lower octave with great finesse and feel. I had trouble nailing it - I sounded awkward and sloppy. I wasn't able to make that jump and then come back down smoothly and elegantly and continue the bassline seamlessly. (Now, many years and practice sessions later, I can play it like Macca did... almost...)
So, I had a choice: try to play it like the record and sound sloppy, or play that b3 on the lower octave and sound good. I chose the latter - made sure to play it with just the right feel and as good as sound as I could get, and the guys in the band were OK with that - it worked. It didn't produce the wonderful Beatles' sound (we weren't the Beatles anyhow...) but it was 'passable': the song's identity was clear - the bass part was recognizable and functionally sufficient, because I faked it well enough: I played what I could play credibly and captured something of the original, based my own skill-set at the time.
Obviously, this is only for now - to get you through what appears to be this difficult initial period. As time goes on, you'll become more familiar with the material and will be able to do more with it - nail the parts perfectly, and add your own individual touches to it too. You may even develop your own original approach to a song that works better, or turns it in a different direction that your new band-mates would enjoy- that's quite common among working, growing musicians.