Cell type specific gene expression programs are dictated by transcription factors, which first recognise and then regulate the expression of their target genes. The last two decades have seen major advances in our understanding of how transcription factors recognise specific genes through cis-regulatory elements, such as promoters and enhancers. However, the principles and molecular mechanisms by which they actually regulate their targets remain poorly understood. To help address this challenge, we coupled Gal4 transactivation assays with comparative CRISPR-Cas9 screens, enabling comprehensive assessment of the proteins required for transactivation by 9 different transcription factors in human cells. Using this system, we are able to associate the requirement of specific transcriptional regulators with particular transcription factors, classify transcriptional regulators as generic or variable, identify transcriptional co-dependencies and functionally dissect the critical Mediator and SET coactivator complexes. By comparing and contrasting the proteins required by different TFs in a controlled context, we can provide the first global insights into the specificity and heterogeneity of transactivation. Overall, we observed substantial diversity in the proteins required by different transcription factors, which suggests that mammalian transcription factors do not function as simple on-off switches and instead transmit more complex regulatory information to their target genes by using specific mechanisms of activation.